1. Field of the Invention
This invention relates to an interface circuit installed for an interface between a subscriber line and telephone terminal equipment, and more particularly to technological advances that reduces the dead zone characteristics of a diode bridge and improves the characteristics of direct current in an interface circuit.
2. Description of the Related Art
FIG. 4 shows a conventional telephone terminal equipment interface circuit 101. In FIG. 4 relay L is closed when the telephone is off the hook and line current that flows to the TIP terminal and the RING terminal of the subscriber line, is supplied to a call transmission/reception circuit 10 after being rectified at a diode bridge 20 comprising the diode elements D11, D12, D13 and D14.
If however, the diode elements D11, D12, D13 and D14 are composed of silicon diodes for example, the silicon diodes will not conduct if the line current supplied from the subscriber line is weak, because a voltage of 0.6 V is required to cause such diodes to conduct; thus, line current would hardly be supplied to the call transmission/reception circuit 10. In the example shown in FIG. 4 the two diode elements D11 and D14, and D13 and D12 are serially connected in the forward direction therefore, a range of 0 V to 1.2 V (=0.6 V×2) exists as a dead region. The existence of this dead region causes obstruction to the function of the call transmission/reception circuit 10.
Further, if the transmission/reception circuit 10 is designed using a diode bridge 20 with such a dead region, the direct current resistance of the interface circuit 101 must be set low in order to comply with the current-voltage characteristics laid down in the U.S. EIA-470B standards. If the direct current resistance of the interface circuit 101 is set low, excessive loop current flows into the call transmission/reception circuit 10 when there is a presence of lightning surge current in the subscriber line, thereby causing damage to equipment.